The present invention relates to a fixing device for heating and pressing a sheet with a toner image, and fixing the toner image onto the sheet, and an image forming apparatus provided with the aforementioned fixing device.
In the conventional art, as the fixing device used in the image forming apparatus using electrophotographic process in a photocopier, printer, fax machine, and multifunction machine provided with the functions thereof, the fixing device of heating roller system (also referred to as a heat-fixing roller system) has been employed over an extensive range from low-speed to high-speed machines as well as from monochromatic to color machines. In the fixing device of heating roller system, heat and pressure are applied to the transfer material with an unfixed toner image formed thereon, by a fixing nip portion made up of a heating roller kept at a predetermined temperature, and a pressure roller having an elastic layer and being pressed against the heating roller, while the transfer material is sandwiched and conveyed thereby.
Widening the fixing nip portion is required to heat toner efficiently on the sheet surface by the demand for colorization and speedup in late years. In this case, viewing the constitution of the aforementioned fixing device of a heat roller system, a means for enlarging the diameters of the two rollers or increasing the pressurizing force between the rollers, thereby increasing the crush (distortion) amount of the rollers would be considered to widen the nip portion. However, when such a method is employed, there may arise a problem with respect to the enlargement of the size of the fixing device and the reduction in durability of the fixing device and the degree of freedom of design conditions is low.
As a method for solving this problem, a fixing device of a belt nip system having an rotating endless fixing belt driven by a roller and a pressing member fixed on the inner circumferential surface side of the fixing belt and pressing the fixing belt toward the heating roller by the pressing member has been adopted in recent years. In the fixing device of a belt nip system, from the viewpoint of its constitution, the width of fixing nip portion can be set to be wider than in the heating roller system.
The fixing device of nip belt system is designed in such a way that the fixing belt is pressed against the heating roller by means of a pressure member of fixed sliding type. Accordingly, if there is a great friction between the inner surface of the fixing belt and the pressure member, traveling of the fixing belt is blocked, and this will cause image misregistration or sheet wrinkles.
In an attempt to solve such problems, Unexamined Japanese Patent Application Publication No. 2002-148970 discloses a fixing device to prevent image misregistration from occurring, wherein the aforementioned fixing device is provided with a sheet-shaped member to cover the pressure member for the purpose of reducing the drag of friction with the inner circumferential surface of the fixing belt, and large irregularities are formed on the surface of the sheet-shaped member.
In the fixing nip portion of the fixing device of the belt nip system, the pressure member and a pressure roller located on the downstream side thereof are pressed against the heating roller, whereby a fixing nip portion of relatively greater width is formed. Further, the pressure roller transforms the elastic layer of the heating roller, and thereby facilitating separation of sheet from the heating roller. In the fixing nip portion of such a fixing device, when there is a wider low-pressure area between the pressure roller and pressure member wherein the pressure is low, the failure is more frequently caused by image irregularities. The failure in the sense in which it is used here refers to the image defect such as irregularities of the toner image caused by expansion of air and production of vapor in the toner layer as a result of heating in a low-pressure area.
To solve the aforementioned problems, the Unexamined Japanese Patent Application Publication No. H11-2979 discloses a fixing device including a pressure member equipped with a projection which protrudes toward the pressure roller side, and is arranged so as to press against the endless belt surface leading to the pressure roll. This is intended to improve the pressure distribution.
To prevent an image defect from occurring due to the low-pressure area of the fixing nip portion, it is effective to allow the pressure member to come as close as possible to the pressure roller in order to reduce the width of the low-pressure area. However, when the pressure member is brought close to the pressure roller, a sliding sheet covering the pressure member is pulled toward the pressure roller by the sliding with the fixing belt, and is pulled inside the pressure roller brought close thereto, if the sliding sheet is slacked. FIG. 12 shows that the sliding sheet is pulled inside the pressure roller. As shown in FIG. 12, when the sliding sheet 860 is slacked, the sliding sheet will be pulled inside the contact surface “ci” between the pressure roller 84 coming close on the downstream side of the sheet and fixing belt 82. When the sliding sheet has been pulled inside, wear will progress, with the result that the sliding sheet will be broken.
To prevent this, the pressure member must be covered with the sliding sheet in such a way that the sliding sheet is not slacked. However, if the sliding sheet is too tight, transformation of the pressure member will be blocked when the pressure member is pressed, and a desired pressure cannot be applied, with the result that pressure failure will occur. To solve this problem, a technique was proposed wherein the degree of slack was set in such a level that the sheet would not be pulled inside the pressure roller, and transformation of the pressure member would not be blocked.
However, the aforementioned method was accompanied with a new problem wherein, on the end part outside the pressure member, the sliding sheet was pulled inside the pressure roller and was broken. The cause of this problem can be explained as follows: On the end part, there is no contact with the pressure member, and therefore, there is much slack of sliding sheet and the pressure roller tends to pull it inside. Further, frictional drag between the pressure member and sliding sheet serves to reduce the force of pulling inside, but it does not work on the end part since there is no pressure member. Thus, the sheet tends to be pulled inside the pressure roller.
The present invention was created in order to solve these problems in the conventional art. An object of the present invention is to provide a fixing device using a fixing belt to ensure that the sliding sheet covering the pressure member that is pressed against the inner circumferential surface of the fixing belt will not be pulled inside the nearby roller to be broken.